1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device for stabilizing a texture of liquid crystal molecules, and maximizing luminance.
2. Discussion of the Related Art
In recent years, the development of information society increases demands for various display devices, so that many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some types of the flat display devices are already used as displays of various equipment.
Among the various flat display devices, the liquid crystal display (LCD) device has been most widely used due to its advantageous characteristics of thinness, lightness in weight, and low power consumption, whereby the LCD device has come to replace Cathode Ray Tube (CRT). In addition to the mobile type LCD devices such as a display for notebook computers, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology, research in improving the picture quality of LCD devices has been in some respects lacking as compared to other features and advantages of the LCD devices. Therefore, in order to use LCD devices in various fields as general displays, it is preferable that the LCD device can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thinness, and low power consumption.
The LCD device includes an LCD panel for displaying an image and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other at a predetermined interval and a liquid crystal layer injected between the first and second glass substrates. The liquid crystal layer is driven by an electric field generated between the first and second glass substrates, thereby controlling light transmittance by the liquid crystal layer. As a result, an image is displayed on the LCD panel.
Among the LCD devices, a Twisted Nematic (TN) mode LCD device, in which longitudinal directions of liquid crystal molecules between the lower and upper substrates are parallel with the lower and upper substrates, has been most generally used. In TN mode, the liquid crystal molecules are spirally twisted with a predetermined pitch.
The TN mode LCD device has characteristics of varying transmittance of light at each gray level depending on viewing angles. Specifically, the transmittance of light is distributed symmetrically in right and left directions of the TN mode LCD device, but asymmetrically in lower and upper directions.
In order to overcome such a problem, a method is proposed to compensate for the viewing angle dependency by dividing pixels into multiple domains with each domain having a different alignment direction of the liquid crystal layer in a Vertical Alignment (VA) mode. In the VA mode, an auxiliary electrode or an electric field inducing window is formed on the upper substrate to distort an electric field, thereby obtaining a wide viewing angle.
A related art LCD device will be explained with reference to the accompanying drawings. FIG. 1 is an enlarged plan view illustrating a related art LCD device, and FIG. 2 is a cross-sectional view illustrating a related art LCD device taken along line I-I′ of FIG. 1.
As shown in FIG. 1 and FIG. 2, the related art LCD device includes lower and upper substrates 1 and 10 facing each other, and a liquid crystal layer 16 between the lower and upper substrates 1 and 10.
The lower substrate 1 includes a plurality of gate and data lines 2 and 4 crossing each other to define a plurality of pixel regions. A gate electrode 2a extends from both sides of the gate line 2. The lower substrate 1 also includes a gate insulating layer (not shown); an active region 3 on the gate insulating layer above the gate electrode 2a; a pixel electrode 7 in the pixel region in the same layer as the active region; a source electrode 4a extending from the data line 4 and overlapping one portion of the active region 3; a drain electrode 4b spaced apart from the source electrode 4a and overlapping the other portion of the active region 3 and a predetermined portion of the pixel region; an interlayer insulation film 6 on the entire surface of the lower substrate 1 including the pixel electrode 7; an orientation control electrode 5 on the interlayer insulation film 6 overlapping the periphery of the pixel electrode 7; and a first alignment layer 8 on the lower substrate 1 including the orientation control electrode 5.
The upper substrate 10 includes a black matrix layer (not shown) that prevents light leakage at the regions other than the pixel regions of the lower substrate 1, a color filter layer (not shown) on the upper substrate 10 corresponding to the black matrix layer of the upper substrate 10 and the pixel regions of the lower substrate 1, a common electrode 13 on the color filter layer, the common electrode 13 having an X-shaped orientation control window 14, and a second alignment layer 15 on the upper substrate 10 including the common electrode 13.
Although not shown, first and second polarizing plates are formed on a lower surface of the lower substrate 1 and an upper surface of the upper substrate 10, respectively. The first and second polarizing plates have polarizers crossing each other.
At this time, the orientation control electrode 5 is connected to another orientation control electrode of an adjacent pixel. Also, the orientation control window 14 is used for distorting the vertical electric field formed between the pixel electrode 7 and the common electrode 13.
That is, when the electric field is generated between the pixel electrode 7 of the lower substrate 1 and the common electrode 13 of the upper substrate 10, as indicated by the arrows of FIG. 2, a fringe field is generated by the orientation control window 14 inside the common electrode 13. Thus, liquid crystal molecules are differently oriented at both sides of the orientation control window 14 by the fringe field, thereby compensating a viewing angle.
Also, the liquid crystal layer 16 has a negative dielectric anisotropy. In the. VA mode using the fringe field, if an additional structure for inducing a fringe field (such as a side electrode) is not formed, a texture of liquid crystal molecules varies at each pixel region, thereby deteriorating uniformity of the LCD device.
Meanwhile, as shown in FIG. 3, devices having a liquid crystal layer 16 (FIG. 2) having negative dielectric anisotropy between the lower and upper substrates can have black lines on the screen generated by longitudinal axes of the liquid crystal molecules 21 arranged in parallel to the optical axes of the first and second polarizing plates.
In order to solve such a problem, chiral dopant is added to the negative dielectric anisotropy liquid crystal (vertical alignment liquid crystal with chiral dopant, hereinafter, referred to as VAC) as illustrated in FIG. 4. That is, light is transmitted at the portion where the longitudinal axes of the liquid crystal molecules 21 are arranged in parallel (arrow direction) to the optical axes of the first and second polarizing plates. Thus, luminance is improved as compared with a case using the general liquid crystal layer having the negative dielectric anisotropy. Accordingly, if the general liquid crystal layer having the negative dielectric anisotropy is used, ‘A’ and ‘B’ portions of FIG. 3 become black as a result of the liquid crystal molecules arranged parallel to the optical axes of the first and second polarizing plates.
Meanwhile, using the VAC, the liquid crystal molecules are arranged in a twisted structure, as illustrated in FIG. 4 according to helical characteristics of the chiral dopant. Thus, the light is transmitted at ‘C’ and ‘D’ portions of FIG. 4, thereby improving luminance.
FIG. 5 illustrates the luminance when a general liquid crystal layer having the negative dielectric anisotropy is used as compared to the luminance when the VAC is used. If the VAC (dotted line) is used for the related art LCD device, luminance is improved as compared with the case using the general liquid crystal layer having the negative dielectric anisotropy without chiral dopant (solid line).
However, the related art LCD device has the following disadvantages.
In the related art LCD device, the orientation control window is formed to obtain the wide viewing angle, whereby the texture of the liquid crystal molecules is stabilized. However, in case of using the liquid crystal layer having the negative dielectric anisotropy, luminance is deteriorated because the liquid crystal molecules are arranged in parallel with the first and second the polarizers.
In case of using the VAC having the liquid crystal layer of the twisted structure, luminance is improved. However, it is hard to maximize luminance due to the thin color filter layer on the upper substrate.